1. Field of the Invention
The present invention relates to an epitaxial wafer for a semiconductor light emitting device, a method for fabricating the same and a semiconductor light emitting device, more particularly, to an epitaxial wafer for a semiconductor light emitting device (such as a light emitting diode, a semiconductor laser), which includes aluminum gallium indium phosphide (AlGaInP) based material using zinc (Zn) or magnesium (Mg) as a p-type dopant, a method for fabricating the same, and a semiconductor light emitting device fabricated by using the epitaxial wafer for a semiconductor light emitting device.
2. Description of the Related Art
In recent years, the high density optical disk apparatus using an AlGaInP based visible light semiconductor laser as a light source has been positively developed among the semiconductor lasers. The Fabry-Perot type laser diode (LD) used for this light source has a layered structure in which at least a n-type AlGaInP cladding layer, an active layer, a p-type AlGaInP cladding layer, and a p-type GaAs cap layer are sequentially laminated on a n-type GaAs substrate by using the Metalorganic Vapor Phase Epitaxy method (MOVPE method). If necessary, a n-type GaAs buffer layer, and further a n-type GaInP buffer layer may be interposed between the n-type GaAs substrate and the n-type AlGaInP cladding layer. Further, if necessary, a p-type GaInP intermediate layer may be interposed between the p-type AlGaInP cladding layer and the p-type GaAs cap layer. Still further, if necessary, an epitaxial wafer having a configuration in which a GaInP layer is interposed into a part of the p-type AlGaInP cladding layer may be used for the etching control in the process and the design of a refractive index.
For a light source for reading/writing in a high density optical disk apparatus, a stable operation with high power at a high temperature required. Accordingly, it is necessary to increase a carrier concentration of the p-type cladding layer to provide a high carrier concentration. Zn or Mg may be used as the p-type dopant satisfying this request. Japanese Patent Laid-Open No. 11-186665 discloses an example in which Zn is used as the p-type dopant for the p-type cladding layer.
However, there is a problem in that the dopant is diffused from the p-type cladding layer to the active layer. When the diffused amount of the dopant is much, a critical defect for a function as a semiconductor laser device will be caused. Herein, Zn is relatively easily diffused while Mg is hardly diffused. Therefore, the recent tendency is to form the p-type cladding layer by using Mg having a small diffusion constant in comparison with Zn. Since Mg is hardly diffused compared with Zn, the carrier concentration of the p-type cladding layer can be made higher by doping Mg.
On the other hand, from the need to decrease a contact resistance of an electrode as low as possible, the carrier concentration of the p-type cap layer (contact layer) should be increased to provide a high carrier concentration. This p-type cap layer is generally made of gallium arsenide (GaAs), and Zn is used as a dopant for realizing a high carrier concentration, since the carrier concentration of the p-type cap layer should be higher by 1 digit than that of the cladding layer, as disclosed by Japanese Patent Laid-Open No. 11-186665.
In addition, it is proposed by Japanese Patent Laid-Open No. 2002-261321 that a diffusion control layer is interposed between the p-type cladding layer and the active layer, and the diffusion control layer is mainly composed of a GaAs-based compound semiconductor and contains carbon (C) of a concentration of 5×1018/cm3 to 1×1020/cm3. As for this, the carbon (C) in the diffusion control layer becomes a barrier and the dopant diffusion of Zn or Mg doped in the p-type cladding layer or the active layer can be effectively controlled.
As shown in the conventional arts such as Japanese Patent Laid-Open No. 11-186665 and Japanese Patent Laid-Open No. 2002-261321, the attention has been paid to the prevention of a disadvantage in that Zn in the p-type cladding layer enters into the active layer. For example, in Japanese Patent Laid-Open No. 2002-261321, the diffusion control layer containing the carbon (C) is interposed between the p-type cladding layer and the active layer. The diffusion control layer is a layer for absorbing Zn diffused from the p-type cladding layer so as to reduce the amount of Zn diffused in the active layer.
However, as described below, it is necessary to prevent Zn of the p-type cap layer from entering into the p-type cladding layer and the active layer.
In more concrete, if the p-type cap layer is doped with Zn with a high concentration, Zn will be rapidly diffused into lower layers during the epitaxial growth. In other words, Zn will be diffused into the layers located nearer to the substrate. When the p-type cladding layer provided beneath the p-type cap layer is doped with Zn, Zn in the p-type cladding layer will be pushed out and diffused into the active layer (push-out diffusion). On the other hand, when the p-type cladding layer is doped with Mg, Zn in the p-type cap layer will be diffused at a burst into the active layer by a mutual diffusion. For either case, there are disadvantages in that a half bandwidth of a photoluminescence spectrum (hereinafter, referred as “PL half bandwidth”) of the active layer becomes greater, or that an emission intensity is decreased. In other words, damages for a crystal quality of the active layer due to the diffusion of Zn causes the elevations of a threshold current and a operating current and the deterioration in product reliability.